Phase transformation at the surface of TiO2 single crystal irradiated by femtosecond laser pulse

Phase transformation of a titanium oxide crystal irradiated by a femtosecond laser from rutile to anatase was studied by Raman spectroscopy. In the case of the high temperature phase of TiO2 single crystal rutile, irradiated by the 120 fs, 800 nm, 250 kHz femtosecond laser with an average power of 300 mW for a short time, the intensity of Raman active mode Eg (446 cm-1) of TiO2 would decrease, while that of A1g (611 cm-1) increased, which indicated the color-center-defect-cluster was formed. After the longer irradiation time (less than 600 s), four new Raman active modes would occur, so a part of rutile in the irradiated region was transformed into anatase phase. As the irradiation time increased, the component of anatase increased to a constant, while that of rutile decreased. By this means, we can selectively induce anatase on the rutile surface through controlling the femtosecond laser exposure region. We suggest that this technique can be applied to fabricate micro patterns of anatase.